1. Technical Field of the Invention
The present invention relates generally to a vehicle speed projecting system and a tire condition monitoring system using same to measure properties of the tire, for example, the inflation pressure, the type of tire, or the amount of tire wear.
2. Background Art
There has been proposed various warning devices designed to measure a certain property of the tire. For example, an inflation pressure detecting system is well known in the art wherein the internal pressure of the tire is measured indirectly using wheel speed of each tire based on the fact that the tire radius is changed due to a drop in inflation pressure. In this prior art system, when a wheel speed of a specified wheel exceeds that of another wheel, the system concludes that the tire radius has been decreased due to the drop in inflation pressure.
The above prior art system, however, encounters a drawback in that since a variation in tire radius, or a drop in inflation pressure is measured by comparing wheel speeds with each other, it is difficult to maintain a desired measurement accuracy. For example, when the inflation pressure of only one wheel drops, the pressure drop is detected faithfully, however, when the inflation pressures of all tires are decreased gradually, this is difficult to detect.
In addition, when a studless snow tire is mounted on the vehicle in combination with normal tires or a tempa spare tire is attached to only one wheel, a similar problem is raised.